1. Field of the Invention
The invention relates to the tempering of glass sheets, and optionally their simultaneous bending by the so-called contact process and it relates more particularly to the particular treatment of the edges of the glass sheets during this tempering, optionally accompanied by a bending.
2. Background of the Related Art
In a tempering of glass in air, i.e., by blowing cold air on a glass sheet heated to its tempering temperature, the cooling is the greatest and the heat exchange is the best on the edges of the glass. There result, on the edges of glass tempered in air, compression stresses on the entire thickness of the glass and up to a certain distance from the edges that are currently called "edge stresses." These stresses assure the strength and stability of this glass sheet, particularly during operations such as its mounting in an automobile window opening. Moreover, an impact applied to the edge of a glass sheet tempered in air generally causes only chipping. On the other hand, the same impact applied to the edge of a contact-tempered glass sheet can cause shattering. This is due to a fault inherent in the contact tempering process itself since the edge of glass is not cooled during the contact tempering.
Actually, in this contact tempering process, a glass sheet is cooled using cooling plates applied against all of its faces, either directly, or preferably with insertion of a heat and mechanical buffer material.
Thus along the faces of this gass sheet, cooling is facilitated by contact with the cooling plates or with the buffer material, but on the edge of this glass sheet this contact does not exist and edge stresses result that are too small in absolute value and/or exist over too small a depth, which is reflected by a fragility of the glass sheet thus tempered to any impact on its edges.